ENH: handle uniform Su/Sp/SuSp source terms directly

- avoid any operations for zero sources

- explicit sources are applied to the entire mesh can be added directly,
  without an intermediate DimensionedField

- update some legacy faMatrix/fvMatrix methods that used Istream
  instead of dictionary or dimensionSet for their parameters.
  Simplify handling of tmps.

- align faMatrix methods with the updated their fvMatrix counterparts
  (eg, DimensionedField instead of GeometricField for sources)